There is a growing demand for battery systems which are intended to be used in fixed applications such as wind power installations and emergency power systems or else in vehicles. All these demands result in stringent requirements in terms of the reliability and fail safety. The reason for this is that complete failure of the voltage supply provided by the battery system can lead to a failure of the entire system. Therefore, in wind power installations batteries are used in order to adjust the rotor blades when the wind is strong and in this way to protect the installation against excessive mechanical loads which can damage or even destroy the wind power installation. If the battery of an electric car were to fail, the car would become undrivable. In turn, an emergency power system is intended in fact to ensure uninterrupted operation of, for example, a hospital, and therefore itself cannot possibly fail.
In order to be able to make available the power and energy which is required for the respective application, individual battery cells in a battery module are connected in series and partially additionally in parallel, wherein a high overall voltage is produced which constitutes a source of hazards and has to be protected. Therefore, two main contactors are usually provided, which disconnect the positive and the negative poles of the battery module in response to a corresponding control signal and in this way connect the battery module to the outside in a voltage-free fashion.
In order to monitor the battery cells, a so-called control unit or battery control unit (BCU) is used which has a microcontroller as a central component. The BCU is operated in a low voltage network and therefore has to be isolated from the high voltage network with the battery module. However, at the same time the microcontroller of the BCU must receive current measurement data from the battery cells (or cell monitoring units which are arranged in the battery module and which measure the various battery parameters), in order, for example, to determine the state of charge or state of ageing thereof or the overall voltage of the battery module, with the result that communication has to be possible between the high voltage network and the low voltage network despite the requirement for isolation. Since the cell monitoring units are connected in terms of potential to the high voltage network, but the microcontroller of the BCU is connected to the low voltage network, the cell monitoring units are typically connected to the microcontroller via an isolation module or isolator. FIG. 1 and FIG. 2 show examples of such an arrangement.
In the example in FIG. 1, an isolator 14 is arranged in the high voltage network and is connected to a cell monitoring unit 12-1 of a plurality of cell monitoring units 12-1 to 12-n. The cell monitoring units 12-1 to 12-n are each connected to a battery cell or a group of battery cells 11-1 to 11-n. A communication bus, to which all the cell monitoring units 12-1 to 12-n are connected, is made to extend via the isolator 14 and from the high voltage network into the low voltage network to the microcontroller 15. The microcontroller 15 is connected to two main contactors 16-1 and 16-2, which, in response to a control signal of the microcontroller 15, disconnect the battery cells 11-1 to 11-n or connect them to a load. The microcontroller 15 is connected to further electronic units via a Control Area Network (CAN) bus 17 or a similar communication line. Alternatively, the isolator 15 can be arranged in the low voltage network, for example on the BCU, as is shown in the example in FIG. 2, wherein identical reference signs denote identical components.
The two solutions shown in FIGS. 1 and 2 have the disadvantage that the isolator 14 entails high costs because it usually has to be implemented as an SPI bus isolator.
FIG. 3 shows a further previously known solution in which an additional microcontroller 13 is integrated into a cell monitoring unit 12-1, said microcontroller 13 communicating with the microcontroller 15 of the BCU via an isolator 14 which is arranged in the low voltage network. This solution has the disadvantage that one of the cell monitoring units has to be configured differently than the cell monitoring units for the remaining battery cells, but on the other hand the abovementioned problem of the high costs for the isolator 14 is eliminated here since a CAN bus can be used for communicating over the boundary between the high voltage network and the low voltage network. However, using a CAN bus has in turn the disadvantage that the synchronization of the battery voltages measured by the cell monitoring units in the high voltage network with the measurement of the battery current which is usually carried out in the low voltage network is difficult owing to the communication via the CAN bus.
In addition, complex safety mechanisms, which ensure that the microcontroller 15 functions correctly at all times since the Li ion batteries used constitute a considerable potential hazard, have to be provided in all three examples of previously known battery systems.